The delivery of a therapeutic compound to a subject is important for its therapeutic effects and is usually impeded by a limited ability of the compound to reach targeted cells and tissues. Improvement of such compounds to enter the targeted cells of tissues by a variety of means of delivery is crucial.
Examples of biologically active molecules for which effective targeting to a patient's tissues is often not achieved include numerous proteins including immunoglobulin proteins, polynucleotides such as genomic DNA, cDNA, mRNA, and siRNA, antisense polynucleotides; and many low molecular weight compounds, synthetic or naturally occurring, such as the peptide hormones and antibiotics.
Efficient delivery to cells in vivo requires specific targeting such as provided by conjugating a targeting ligand to the biologically active molecule. The targeting ligand provides specificity by assisting in receptor binding at the required target cell or tissue. A targeting ligand can also mediate receptor-mediated endocytosis at the target site by which the biologically active molecule bound to a membrane receptor is enveloped by the membrane via invagination of the membrane structure or by fusion of the delivery system with the cell membrane. Examples of receptor-mediated endocytotic systems are those that recognize sugars such as galactose, mannose, mannose-6-phosphate; or peptides and proteins such as transferrin, asialoglycoprotein, insulin and epidermal growth factor.
The asialoglycoprotein receptor (ASGP-R) on hepatic cells was identified and characterized on the basis of its ability to bind β-linked galactose or N-acetylgalactosamine (GalNAc) residues on proteins. ASGP-R consists of ASGR1 and ASGR2 subunits forming a variety of multimers that import large molecules across the cellular plasma membrane by endocytosis, a characteristic that makes it a potential target for receptor-mediated drug delivery to hepatocytes and hepatoma cells. A multivalent ligand consisting of several GalNAc molecules can achieve nanomolar affinity. Spacing and orientation among the sugar of the multivalent ligand affects binding. Lipophilic ligands, such as cholesterol or fatty acids can enhance plasma protein binding and consequently circulation half-life, and bind to plasma proteins such as lipoproteins. These ligands also can increase uptake in specific tissues expressing the corresponding lipoprotein receptor.
There remains an unmet need for a receptor-specific multiligand delivery agent and methods for its preparation to improve in vivo delivery of bioactive molecules.